Recently, because optical communication traffic is showing rapid growth, optical modulators that convert an electric signal into an optical signal act as very important key devices. Among the optical modulators, an electroabsorption modulator and a Mach-Zehnder modulator based on a specific material such as compound semiconductor or LiNbO3 have been put to practical use. However, there are problems of expensive manufacturing costs as well as high integration, which are difficult to overcome. As an approach to solve that problem, a Metal Oxide Semiconductor (MOS) optical modulator was proposed in Non-Patent Document 1 by A Liu and seven (7) others, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor” (Nature, Feb. 12, 2004, Vol. 42, p. 615-618). This modulator has merit in that it can be manufactured inexpensively price by a Chemical Metal Oxide Semiconductor (CMOS) process. The MOS optical modulator operates according to the principle that the refractive index of a charge accumulation area is changed by a carrier plasma effect. However, in the optical modulator according to Non-Patent Document 1, a charge accumulation layer is thick in the order of just several tens of micrometers, whereas guiding mode is wide in the order of several micrometers. The result is a low phase modulation effect and a large size (e.g., about 1 cm) since the guiding mode overlaps the refractive index modulation area in a limited range. This as a result causes a problem to the difficulty of high speed operation.
In addition, Patent Document 1 (Japanese Patent No. 2716081) has proposed an optical modulator using a two-dimensional optical waveguide (e.g., a Plasmon waveguide) in which a medium having an electro-optic effect is sandwiched by using metal. This document explains that a small optical modulator can be realized to operate at a very low voltage by setting a two-dimensional optical metal gap waveguide to be sufficiently smaller than the wavelength of incident light. Here, for example, the optical modulator has an operating voltage of 0.1 V, and a length of 6 μm, and the electro-optic medium has a thickness of 6 nm. However, there is a very difficult problem of injecting incident light into the metal gap, which is much smaller than the wavelength of the incident light.
As described above, in the MOS optical modulator described in Non-Patent Document 1, light propagating along the waveguide to which a voltage is applied has a small amount of phase conversion per unit length. Thus, when a Mach-Zehnder interferometer is combined, an increasing length of the waveguide is required in order to increase the modulation ratio by setting the phase difference of two light beams to about π. This as a result causes problems such as an increasing device size of the optical modulator as well as high integration and high speed requirements, which are difficult to achieve.
Furthermore, in the Plasmon waveguide including a waveguide clad made of metal having a complex dielectric constant with a negative real part, propagation loss is generally greater than that of a typical optical waveguide, which is made of core and clad materials having a complex dielectric constant with a positive real part. Therefore, an optical circuit is preferably constructed using the Plasmon waveguide only in the case of phase modulation but using the typical optical waveguide in other cases. In this case, although the Plasmon waveguide and the typical waveguide are required to be coupled with high efficiency, this coupling is difficult to overcome.